Dart: Wait to handle events on a Stub until it makes sense to do it.

mojo/dart/packages/mojo_services/lib/mojo/cookie_store.mojom.dart

Index: mojo/dart/packages/mojo_services/lib/mojo/cookie_store.mojom.dart
diff --git a/mojo/dart/packages/mojo_services/lib/mojo/cookie_store.mojom.dart b/mojo/dart/packages/mojo_services/lib/mojo/cookie_store.mojom.dart
index c6c978cff9269a474018e3683f94d1c56cc6abef..868d987fa91ccf97ec30c51fd1125369c15e4e93 100644
--- a/mojo/dart/packages/mojo_services/lib/mojo/cookie_store.mojom.dart
+++ b/mojo/dart/packages/mojo_services/lib/mojo/cookie_store.mojom.dart
@@ -492,14 +492,19 @@ class CookieStoreProxy implements bindings.ProxyBase {
 
 
 class CookieStoreStub extends bindings.Stub {
-  CookieStore _impl = null;
+  CookieStore _impl;
 
   CookieStoreStub.fromEndpoint(
-      core.MojoMessagePipeEndpoint endpoint, [this._impl])
-      : super.fromEndpoint(endpoint);
+      core.MojoMessagePipeEndpoint endpoint, [CookieStore impl])
+      : super.fromEndpoint(endpoint, autoBegin: impl != null) {
+    _impl = impl;
+  }
 
-  CookieStoreStub.fromHandle(core.MojoHandle handle, [this._impl])
-      : super.fromHandle(handle);
+  CookieStoreStub.fromHandle(
+      core.MojoHandle handle, [CookieStore impl])
+      : super.fromHandle(handle, autoBegin: impl != null) {
+    _impl = impl;
+  }
 
   CookieStoreStub.unbound() : super.unbound();
 
@@ -527,7 +532,9 @@ class CookieStoreStub extends bindings.Stub {
                                                           0,
                                                           message);
     }
-    assert(_impl != null);
+    if (_impl == null) {
+      throw new core.MojoApiError("$this has no implementation set");
+    }
     switch (message.header.type) {
       case _cookieStoreMethodGetName:
         var params = _CookieStoreGetParams.deserialize(
@@ -582,10 +589,23 @@ class CookieStoreStub extends bindings.Stub {
 
   CookieStore get impl => _impl;
   set impl(CookieStore d) {
-    assert(_impl == null);
+    if (d == null) {
+      throw new core.MojoApiError("$this: Cannot set a null implementation");
+    }
+    if (isBound && (_impl == null)) {
+      beginHandlingEvents();
+    }
     _impl = d;
   }
 
+  @override
+  void bind(core.MojoMessagePipeEndpoint endpoint) {
+    super.bind(endpoint);
+    if (!isOpen && (_impl != null)) {
+      beginHandlingEvents();
+    }
+  }
+
   String toString() {
     var superString = super.toString();
     return "CookieStoreStub($superString)";